AI agents call receive_message to retrieve information from Signal MCP without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
This tool retrieves incoming Signal messages. It is a read/fetch operation with no side effects — it does not send, modify, or delete any data. The only concern is privacy (reading potentially sensitive messages), which justifies a low-medium severity, but the action itself is purely Read.
From the tool's definition "Wait for and receive a message using signal-cli"
Documented attack patterns abuse exactly the kind of access receive_message gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Signal MCP, and nothing reaches the server without passing your rules. This is the rule we recommend for receive_message:
{
"version": "1",
"default": "deny",
"tools": {
"receive_message": {}
}
}receive_message is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.
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Wait for and receive a message using signal-cli. It is categorised as a Read tool in the Signal MCP MCP Server, which means it retrieves data without modifying state.
Register the Signal MCP server in PolicyLayer and add a rule for receive_message: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Signal MCP. Nothing to install.
receive_message is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the receive_message rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for receive_message. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
receive_message is provided by the Signal MCP server (rymurr/signal-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Signal MCP, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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3 Signal MCP tools catalogued and risk-classified — across an index of 43,000+ MCP servers.